Open fields: bad. You run and run and run and shoot. Nothing to hide behind.
Open field with trees: bad. You end up hiding behind one tree, static, and running to the next tree. Repetition = boredom.
Open field with half walls: good. Allows for unseen approaches and defensive posturing.
Tunnels: good. You can run in them, you can't see who's ahead of you, and you can't fire until the end. Heightens the suspense and increases the adrenaline.
Staircases: good. Allows for grenades to be lobbed up and down, which is always fun.
There seems to be a correlation between not being seen and fun. The less you see of your opponent, the more fun it is. How can I incorporate all these elements into my game field tomorrow...the tarps and cardboard boxes will help, but I really would need to have the tunnels for max fun. Thankfully the trees provide this.